This invention relates to a cable structure suitable for a long haul submarine optical fiber cable in the field of optical communications employing optical fibers of low loss as a transmission medium.
Optical fibers have such features as a low loss, a wide transmission band, noninductivity and a light weight, and hence are very advantageous as a wide-band transmission medium. Therefore, an attempt has widely been made to achieve large capacity communications with a cable using the optical fibers, and some examples of application of the optical fibers to a submarine cable utilizing its wide transmission band and low loss properties have been reported. These properties are also very desirable for a submarine cable and it is expected that a submarine cable using the optical fibers will be carried into practice. However, since the submarine cable is laid in the surroundings entirely different from those in which a land cable is laid, the following matters must be taken into account in a case of using the optical fibers in the submarine cable.
(1) A cable structure for protecting the optical fibers from the influence of characteristic change due to a high hydraulic pressure and sea water.
(2) A mechanism for providing a tensile strength which is needed in the laying or repair of the cable.
(3) A sheath for protecting the optical cable against various external forces.
(4) A cable structure for power feeding to submerged repeaters.
Optical fiber submarine cables heretofore proposed adopt the construction in which the optical fiber is inserted into a pressure resisting pipe. The optical fiber is made of glass and its characteristics, as an optical transmission line, does not change when being subjected to an external pressure. However, the optical fiber is mechanically weak, so that nylon, polyethylene or like material is coated on the optical fiber to increase its mechanical strength against a tensile force and external forces. The optical fiber thus coated is called an optical fiber core. When the optical fiber core is subjected to an external force, nonuniformity of the coating in the lengthwise direction of the fiber and the adhesion between the fiber and the coating and, in a case of the coating being double or triple, nonuniformity of the adhesion between the coatings in the lengthwise direction of the fiber, cause a very slight bending of the optical fiber, resulting in an increased transmission loss. Therefore, in the laying of the optical fiber core on the seabed, it is necessary to take measures for protecting the optical fiber from the sea water pressure; there has been proposed to insert the optical fiber into a pressure resisting pipe. To further use the pipe as a power feeding conductor or a tension member for increasing the mechanical strength of the pipe against the water pressure and the economization of the cable, the thickness of the pipe must be increased. However, it has heretofore been difficult to continuously manufacture a long thick pipe while at the same time inserting thereinto the optical fiber core. Moreover, the manufacture of the pipe requires heating in the both cases of the extrusion method and the forming method using a roll former, so that if the adiabatic layer for protecting the optical fibers is insufficient, their transmission characteristics are adversely affected. Therefore, it is desirable to produce a thick pipe without involving any heating process. This is difficult with the conventional manufacturing method of pipe. Moreover, also in a case of considering the cable as a whole, the conventional structures are not suitable for a long-distance, deep-sea submarine cable from the economical point of view as well as in terms of handling during cable laying.